Procedure for surface treatment of iron and steel

ABSTRACT

A PROCESS FOR TREATING IRON OR STEEL SURFACES COMPRISING THE STEPS OF: (1) CLEANING THE METAL SURFACES WITH AN AQUEOUS ALKALINE SOLUTION, (2) CONTACTING THE CLEANED METALLIC SURFACES WITH AN AQUEOUS SOLUTION HAVING A PH BETWEN 4 AND 5.5 AND CONTAINING LACTIC ACID AND AN NON-IONIC WETTING AGENT, (3) APPLYING A PHOSPHATE DEPOSITE TO THE METALLIC SURFACES AND, (4) APPLYING AN ORGANIC DEPOSIT TO THE THUS TREATED METALLIC SURFACES WITH AN ELECTRO-IMMERSION PROCESS.

United States Patent O 3,795,548 PROCEDURE FOR SURFACE TREATMENT OF IRON AND STEEL Hans Hansen, Karhen, Peter Jorns and Josef Rones, Frankfurt am Main, and Wolfgang Wuttke, Erzhausen, Germany, assignors to Oxy Metal Finishing Corporation, Warren, Mich. No Drawing. Filed Jan. 10, 1972, Ser. No. 216,722 Int. Cl. C23f 7/10 US. Cl. 148-615 R 3 Claims ABSTRACT OF THE DISCLOSURE A process for treating iron or steel surfaces comprising the steps of:

(1) cleaning the metal surfaces with an aqueous alkaline solution,

(2) contacting the cleaned metallic surfaces with an aqueous solution having a pH between 4 and 5.5 and containing lactic acid and an non-ionic wetting agent,

(3) applying a phosphate deposit to the metallic surfaces and,

(4) applying an organic deposit to the thus treated metallic surfaces with an electro-immersion process.

BACKGROUND This invention refers to the surface treatment of workpieces made of iron and steel, and particularly to an improved procedure during the chemical pretreatment of the workpieces for applying an organic deposit with the electro-immersion process.

In order to prepare iron and steel for the electro-immersion lacquering, it is common to clean the workpieces, to rinse one or several times with water and to phosphate them. For the cleaning operation usually mild alkaline cleaners with a pH-range from 8 to 11 are used. However, in practice sometimes difficulties occur with these cleaners, because they do not always completely remove corrosion protection oils and compounds from the metal surface, particularly when the workpieces have been in storage for a long period. Due to the remaining oily and greasy impurities, the workpieces will not be wetted at all or only partly with water after rinsing. In these cases, an uneven phosphate deposit will result after phosphating that has an unfavorable eifect upon the electro-immersion process.

This problem of uneven phosphating is especially disturbing when one treats the workpieces in continuous spraying equipment. In this process the workpieces, either hanging on a conveyor or set up on an equalizing truck, are passed continuously through each single treatment zone of the equipment. Between the spraying nozzles of each treatment zone there are transition distances with dimensions that possibly prevent a mixing of the different solutions. Depending upon the shape of the parts to be treated and the transportation speed, the intervals between the rinsing zone and the phosphating zone range for instance from 15 to 60 seconds. In this transition zone a reaction of the phosphating solution on the metal surface cannot be completely avoided. Due to maladjusted nozzles, or deviated spraying jets, or fine dispersion of the phosphating solution, the metal surface will be hit in the transition zone. Depending upon the induction time, more or less strong prepassivations will result, which cause an uneven development of the phosphate deposit in the phosphating zone. There are striations, films and streaklike marks, which may even gleam bluish, in the phosphate deposit. In some cases this unevenness of the phosphate deposit is still ditsinguishable after the electro-immersion lacquering, and, therefore, very disadvantageous and undesirable.

SUMMARY OF THE INVENTION It was discovered that the above difficulties could be eliminated easily as follows. After cleaning with the alkaline solution and before phosphating, the surfaces should be exposed to a weak aqueous acidic solution at a temperature below 60 C. and a pH-value adjusted with lactic acid from 4 to 5.5, preferably 4 to 4.8, that contains at least one non-ionic wetting agent.

DESCRIPTION OF PREFERRED EMBODIMENTS The invented multiple-stage procedure for surface treatment of workpieces made of iron and steel consists consequently of the following steps:

( 1) the workpieces are cleaned with alkaline solution.

(2) the workpieces are exposed to an aqueous solution at a temperature below 60 C. and a pH-value adjusted with lactic acid from 4 to 5.5, preferably 4 to 4.8, containing a nonionic wetting agent.

(3) the phosphate deposit is applied.

(4) the workpieces are coated with an organic deposit in the electro-immersion process.

For cleaning the workpieces, the usual alkaline cleaners can be employed. Their pH-value ranges effectively from 8 to 11. Preferably the alkaline cleaner should contain known additives, as for example titanium ortho phosphate, for the activation of the phosphating procedure. The cleaning can be done by spraying as well as dipping. The cleaner concentration for instance amounts to 3 to 5 g./l. for the spraying method and 20 to 30 g./l. for the dipping method. An additional advantage is that the cleaner concentration can be reduced in many cases to approximately 50% of the mentioned values, if the above procedure steps are employed. Often the temperature of the cleaning bath can be reduced also.

After the cleaning operation the workpieces can be rinsed in water to avoid dragging cleaner solution into the lactic acid treatment bath. However, the lactic acid treatment can also be performed without water rinse in between in order to save rinse water. In such a case, the pH-value limitations have to be kept and the bath temperature needs thorough supervision.

If the intermediate acid treatment is being employed after the cleaning process with alkaline, it is necessary to adjust the pH-value of the watery solution with lactic acid between 4 and 5.5, preferably up to 4.8. A higher pH- value leads to an uneven formation of the phosphate deposit, which is the next step in the procedure. If a pH- value below 4 is employed, then there is danger of a too strong pickling of the iron surfaces that results in rough crystalline phosphate deposits which are less suitable for an electro-immersion lacquering.

The adjustment of the required pH-value by different inorganic or organic acids, for instance phosphoric acid, amidosulfuric acid, acetic acid, monochloracetic acid, oxalic acid, malonic acid, maleic acid, citric acid, does not lead to the desired advantageous results.

For the preparation of the acid treatment solution lactic acid of normal commercial usage, also of technical quality, and its water soluble salts may be used. Depending upon the hardness of the available water, in general 0.01 to 1 g./l. of lactic acid is required for adjustment of the necessary pH-value. The pH-value adjustment can be measured with a glass electrode or diagram paper. As nonionic wetting agents those products that are normally used commercially may be employed. First of all, they are ethylene oxide adducts to long-chain organic connections with a reactive hydrogen atom, as for instance polyglycol ether of alkyl phenols, fatty alcohols, fatty acid, fatty amines, and fatty acid amines. For the spraying method nonionic and non-foaming wetting agents are employed, and in this method additive compounds of ethylene oxide to polypropylene glycol have especially proved themselves because of their acid resistance and slight foaming. It is possible with these products to prepare sufficiently concentrated lactic acid solutions, and no separation will take place. The appropriate nonionic wetting agent contents in the solution should be at least 0.05 g./l. When using the acid treatment bath, it is possible that the pH-value for instance rises slowly, due to drag-in of the alkaline cleaning solution and a reaction of the acid solution with the iron surface. By intermittent or continuous supplement with a lactic acid wetting agent mixture the desired pH-value can be maintained. Automatically operating pH measure and dose meters can be utilized particularly advantageously in this case.

If too much iron as Well as alkaline accumulates from the cleaning step, then the effect of the acid solution can be degraded, and in such an event, it would be advisable to renew the treatment bath from time to time or to prevent an excessive accumulation of impurity ions by water supply.

The temperature of the acid treatment bath should be below 60 C. Higher temperatures would cause the formation of lactate layers on the workpiece surface, which could degrade the depositing of the phosphate. This is particularly important, in case the lactic acid solution is to be used directly after cleaning with a hot alkaline solution and the workpieces still show a high temperature. In any case, the amount of rinse water for the temperature control is comparatively low, because the workpieces only need to be cooled off to a temperature below 60 C., and is at the same time sufiicient to prevent a disturbing accumulation of impurity ions in the acid treatment solution.

Should, however, no treatment with lactic acid follow after the alkaline cleaning solution, then it will be necessary to use considerably larger amounts of rinse water, in order to prevent dragging alkali remainders into the phosphating bath and mainly to cool the workpieces, thus eliminating an undesired reactive effect upon the workpieces while changing from the rinsing zone to the phosphating zone, which could result in an uneven deposit of the phosphate. In general, the temperature of the rinsing bath is kept at 30 C. requiring relatively large amounts of rinsing water. However, by employing the lactic acid solution treatment it is not necessary to keep the temperature that low during the transition. Due to the higher permissible temperatures, considerable amounts of rinse water can be saved, if the process succession is conducted in agreement with the invention.

If the workpieces contain a lot of solution, due to their shape, or after cleaning with a strong alkaline solution, it would be advisable to perform a water rinse before treatment in the acid solution. This increases the service life of the lactic acid treatment bath, decreases the consumption of chemicals and simplifies the maintenance of the bath.

The usual phosphating solutions based on deposit forming phosphates can be used for phosphating, preferably on zinc phosphate basis. Also the following electro-dip lacquering can be performed in the usual way and manner. The invented procedure permits one to obtain in a very simple and safe way thin, finely crystalline and hard phosphate layers that do not exhibit streaks or marks of any kind and, therefore, are extremely suitable for the electro-dip finishing so that a first class quality end product is obtained.

This surprisingly advantageous result was quite unexpected, due to the state of the technique. According to the procedure of the German Pat. 862,348, alkaline cleaned surfaces should be rinsed with an acid solution at a pH-value of 4.2 to 6 before phosphating, and the pH- value of the rinsing solution should be adjusted preferably with phosphoric acid or an acid phosphate. This method, however, has the disadvantage of producing prepassivations, which upset the formation of the deposit. According to the procedure of the German Pat. 753,730, the surfaces should be rinsed before phosphating with an oxalic acid solution that may contain a small amount of a strong mineral acid. However, the workpieces become very susceptible to rust, dueto this preliminary rinsing with a very strong acid, which interferes with obtaining good phosphate deposits. It is also known to use a solution of substituted short-chain monocarboxylic acids for the activation and removal of slight rust and scale before phosphating iron surfaces (US. Pat. 3,118,793). The solutions are employed in such concentrations that a strong pickling takes place. During the following phosphating process rough, thick phosphate deposits are ob tained that are unsuitable for the electro-dip lacquering.

The invented process is explained by the following examples and comparative experiments:

Example No. 1

Greased steel plates grade RRST 1405m (as per German Industrial Standard (DIN) 1623, page 1) were pretreated in a continuous spraying plant and then coated with a basic lacquer in the electro-dip process.

In the spraying plant the steel plates were sprayed in the first phase for 2 minutes at 60 C. with an aqueous cleaning solution containing 3.4 g./l. of borax, 0.3 g./l. activating titanium compound, and 0.3 g./l. nonionic and non-foaming wetting agents. In order to come as close as possible to the practice, 3 g./l. of a commercial grade corrosion protection oil (Rustilo 845) were added.

In the second treatment phase the sheets were sprayed for 30 seconds at 45 C. with one of the later mentioned solutions (a) to (i). After a transition time of 30 seconds, the sheets encountered in the third phase a spraying of seconds at 60 C. with a nitrite accelerated zinc phosphate solution. Then the sheets were rinsed with water and dried. Thereafter, the quality of the phosphate layer of the different series was tested.

The solutions that were used in the second treatment phase were prepared by adding 1 g./l. of acid and 0.2 g./l. of the same nonionic and non-foaming wetting agents to tap water (approx. 15 dH) and uniform adjustment to a pH-value of 4.5 with NaOH. The utilized acids are as follows:

(a) Phosphoric acid (b) Amidosulfuric acid (0) Acetic acid (d) Monochloracetic acid (e) Oxalic acid (f) Malonic acid (g) Maleic acid (h) Citric acid (i) Lactic acid The test result showed that only those sheets treated in the solution ('i) (lactic acid) developed a completely faultless phosphate deposit. On the sheets treated with solution (a) (phosphoric acid) had and partly dusty layers with passivations were present. All the other sheets that had been treated with solutions (b) to (h) exhibited a bad development of the phosphate deposit as well as very thick films of rust.

The above sheets were coated using the electro-dip lacquering procedure and as was to be expected by judging the phosphate deposit, only the sheets treated with solution (i) (lactic acid) proved to be suitable.

Example No. 2

Steel sheets were cleaned, as in 'Example 1, and pretreated with a solution containing lactic acid and then phosphated. The pH-value of the solution containing 0.5 g./l. of lactic acid and 0.2 g./l. of wetting agents was adjusted between 3.5 to 6 with NaOH. The pH-value was measured by means of a glass electrode. The influence of the pH-value on the quality of the obtained phosphate deposits can be learned from the following table:

pH-value: Quality of phosphate deposit 6 Streaky, spotted.

5.5 Few streaks and marks.

5.0 Almost flawless.

4.8 Flawless.

4.5 Flawless.

4.0 Flawless.

3.5 No streaks, crystallinity of deposit uneven, partly larger crystals.

During the following electro-dip lacquering only the sheets treated with a pH-range of 4 to 5.5 led to good satisfactory results.

Example N0. 3

In a five phase continuous spraying plant automobile bodies that were contaminated with oil, lubricants, etc. were treated as follows:

(a) 2 minutes of cleaning at 60 C. with a solution containing 3.4 g./l. borax, 0.3 g./l. activating titanium compound and 0.3 g./l. of nonionic and non-foaming wetting agents.

(b) Rinsing 1 minute at approx. 30 C. with tap water,

water supply 8 m. h.

(c) Phosphating 2 minutes at 55 C. with a nitrite catalyst zinc phosphate solution, concentration 14 points (consumption of 0.1 N NaOH at titration of ml. bath sample against phenolphthalein), catalyst contents approx. 100 mg./l. NaNO (d) Rinsing with tap water 1 minute at 35 C.

(e) Rinsing with completely deionized water 1 minute at (f) Spraying with completely deionized water.

The phosphate deposits of the automobile bodies that were treated in the above manner were heavily marked with stripes and spots. Some of the areas that were especially contaminated with dried grease before the above treatment still showed grease remainders after phosphating.

In a second experiment instead of the water rinse before phosphating in step (b) an acid rinse was performed. In this case, the rinsing bath was adjusted with a concentration containing technical lactic acid and weak foaming wetting agents to a pH-value of 4.2 (glass elec trode), which was maintained during this process. The rinsing bath contained approximately 0.18 g./1. of lactic acid and 0.09 g./1. of wetting agents. The water supply was flowing at 1.8 m. /h. The temperature was approximately 48 C. The results from this procedure were very even and fine crystalline phosphate deposits without any stripes or spots. Also, no grease remainders were detected.

(In order to determine the suitability of the automobile bodies treated with both processes for the following lacquering process and pertaining to the corrosion resistance as well as lacquer adhesion, a number of test sheets (steel grade RRSt 1405m) were treated along with the automobile bodies. The test sheets were provided with three layers of lacquer, and the basic lacquer was applied by the electro-immersion process. Then the lacquered sheets were tested in a salt spray test per ASTM B 117-64 and in the so called shot blast test. The latter was conducted as follows:

The sheets to be tested are at first bombarded with 500 grams of steel shots (4.5 mm. squares) in an air flow of 2 atu. (kg./cm. then subjected to the salt spray test 6 per ASTM B 11764 for 120 hours and, thereafter, again bombarded with steel shots. The evaluation is done by comparison with standard sheets of a classification from No. 1 to No. 6, which means that No. 1 has a very good adhesion and No. 6 a completely insufficient adhesion.

The following results were obtained:

Salt spray test Shot blast test Treatment in lacquer flow of lacquer adhesion step (b) 3 mm. in hours classification Water rinse 360 3 Lactic acid rinse 720 1-2 In a 6-phase dipping plant shaped metal parts were assembled to larger units and treated as follows:

(a) Cleaning with an alkaline immersion cleaner at (b) Rinse with cold water. (0) Rinse with cold water. (d) Phosphating with a zinc phosphate solution (28 points) containing chlorate catalyst at 60 C.

(e) Rinse with cold water. (f) Rinse with completely deionized water. (a) y The phosphate deposit of the above mentioned units exhibited strong stripes and marks. In a further series the Water rinse (c) was replaced by a solution containing lactic acid according to the concentration mentioned in Example 3. The phosphate deposits from this second series did not show the defects of the first series. They were even and fine crystalline. Especially at the cavities the units showed a much better deposit formation than in the first series.

Thus, the quality of the electro-dip finished units of the second series was much better.

What is claimed is:

1. A process for treating iron or steel surfaces comprising the steps of:

(1) cleaning the metal surfaces with an aqueous alkaline solution;

(2) contacting the cleaned metallic surfaces with an aqueous solution having a pH between 4 and 5.5 and containing about 0.01 to l g./l. lactic acid and non-ionic wetting agent,

(3) applying a phosphonate deposit to the metallic surfaces and,

(4) applying an organic deposit to the thus treated metallic surfaces with an electro-immersion process.

2. The process of claim 1 wherein the solution temperature of step 2 is below 60 C.

3. The process of claim 1 wherein the non-ionic wetting agent in the solution of step (2) is present at a minimum concentration of 0.05 g./l.

References Cited UNITED STATES PATENTS 3,118,793 1/1964 Maloney et a1. 148-6.l5 R 3,454,483 7/1969 Freeman 204l8l 3,467,589 9/1969 Rausch et a1 1486.l5 Z 3,307,979 3/1967 Upham l486.15 Z

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 

